order to obtain satisfactory noise suppression of an automobile engine over the entire range of engine speeds, noise suppressors have been designed that change the way in which they suppress noise according to the engine speed.
For example, Tokkai Sho 64-60709 published by the Japanese Patent Office in 1989, discloses a muffler provided with a first exhaust pipe suited to reducing exhaust noise in the low engine speed region, and a second exhaust pipe suited to reducing noise in the medium and high engine speed regions. A control valve is provided in the second exhaust pipe that opens only in the medium and high speed regions. In the low engine speed region, exhaust gas is discharged only via the first exhaust pipe. In the medium and high engine speed regions, the control valve opens so that the second exhaust pipe that is suited to reducing noise in the medium and high speed regions is brought into use together with the first pipe. Hence noise is reduced in these regions, while at the same time, the discharge cross-sectional area is increased in the medium and high speed regions where the exhaust gas flowrate is high, and energy losses due to increase of negative pressure are controlled.
However, the control valve in this exhaust device opens and closes via a mechanism consisting of a motor, wire and lever that are activated according to the engine speed. Its construction is therefore complex, and it is costly to manufacture.
Jikkai Sho 57-13832 published by the Japanese Patent Office in 1982, discloses a muffler for a two cycle engine wherein a control valve opens and closes, but according to a different construction that does not use the aforesaid mechanism. In this case, the muffler is divided into two compartments by means of a partition wall. These compartments are linked by two passageways in parallel, and a reed valve that closes one of these passageways under the exhaust pressure is installed adjacent to the upstream compartment.
As this device does not use mechanical force to open and close the valve, its construction is simple. In this case, however, the muffler has only one outlet passage, therefore large energy losses occur in the medium and high speed regions where the exhaust flowrate is high.
If the cross-section of the passage is increased in order to reduce losses, the expansion ratio of the aforesaid compartments becomes smaller so that there is less noise suppression effect. In this case, it is also difficult to effectively cancel the flow noise produced in the reed valve.